1. Field of the Invention
The present invention relates to a method and apparatus for data transmission and particularly to data transmission in which the data is reduced in volume for more efficient transmission.
2. Prior Art
A method of data transmission from one station to another, in which the transmitting station edits the data to be transmitted from time to time, is described in German Patent OS No. 31 10 590, and is used with particular advantage in a remote-control network comprising a tree structure. Remote control and data acquisition systems can have two levels formed by a central receiving station and a plurality of transmitting substations. However, multi-level systems have advantages over a two-level system in that a data reduction can take place in the nodes, involving preprocessing or filtering the data at the nodes. The arrangement of such systems has been relatively difficult, even in conditions of operation where no errors occur, and the monitoring of equipment used in such a system is difficult.
One form of data reduction, using data processing, which can be undertaken in the nodes of a multi-level network involves old-new comparison. For example the computer may send the entire data stream which it manages to one of the nodal points, when any signal status has changed. Subsequently, the new signal status is acknowledged and replaces the old status. This allows the new data stream to be sent only once, but leads to errors and omissions of signals when there is a disturbance of the data in transmission or in acquisition. In order to allow for the possibility of such errors, complicated hand-shakings must be introduced between levels. The system becomes even more complicated when a higher level data processing is employed, in the nodes, for example, suppression of successor reports. The system is further complicated when an attempt is made to provide means for dealing with a failed supply voltage, or to allow the undertaking of repair work on the system.